This is a continuation-in-part of application Ser. No. 610,640, filed Nov. 8, 1990, (now abandoned).
The invention relates generally to the field of semiconductor device packaging and specifically relates to the transfer molding of plastic encapsulated devices. Ceramic hermetic semiconductor packaging is the preferred way of high reliability packaging because it offers several desired advantages Mainly, once sealed, the package is substantially impervious to its environment. The final seal can be accomplished in an environment that is not stressful to the semiconductor device and this environment will be maintained throughout the life of the device. However, such packages have proven to be expensive. In many cases the cost of the package greatly exceeds the cost of producing the chip that it houses. On the other hand, the well known plastic molded semiconductor devices can be produced cheaply. Typically, the chip being housed costs more to produce than the package and this is a desirable economic condition. Plastic encapsulation, while cheap and easy to accomplish, has several disadvantages. For most of these is the problem that such encapsulation is not hermetic. Such packaging allows the long term entry of environmental elements which can adversely affect a semiconductor chip. While this is a difficult problem, the producers of chips have advanced their use of protective seal coatings to a point where hermetic sealing is not necessary. The reliability of plastic encapsulated semiconductor chips in the presence of adverse environments has advanced to a level where hermetic packaging is not always attractive. Certainly the cost/benefit relationship now militates against hermetic packaging. This leaves the device designer with the other problems associated with transfer molding. These include the problems of stress which develops when the plastic encapsulant comes into contact with the semiconductor chip face. Such stress, in the extreme, can result in chip fracture during temperature cycling. Also, some semiconductor chips are sensitive to stress and their operating characteristics will change during encapsulation. Finally, many semiconductor chips require post assembly characterization such as PAL (programmed array logic) devices. In one such heirarchy, fuses are blown to disconnect crossbar switch arrays to produce a desired pattern. With plastic encapsulation, fuse blowing is difficult because the metal vapors thus produced have nowhere to go. Consequently, such programming is done only on cavity-type ceramic packaged devices. It would be desirable to have a cavity type plastic encapsulated package.
In the transfer molding of plastic encapsulated devices, it is common to employ a dambar on the leadframe to control mold flash. The dambar structure is designed to mate with the edge of the cavity in the molding die which creates the final plastic block. After the molding is completed the dambar segments that join the leads together are removed so that the leads are functionally separate. This requires a separate fabrication step and, with the high lead count packages now being favored, can be a difficult task. In fact, when the device dimensions reduce the lead spacing to about 12 mils (0.3 mm) mechanical punches become impractical. At this point, it has become standard practice to use a narrow focussed laser beam to do the cutting. In the square packages, now becoming popular, high lead count packages having 150 to 200 pins result in spacings that make dambar removal very difficult. Accordingly, a modification in the dambar system would be desirable.